Added HTTP Header Content-ID as per RFC 2392

Comment From: pivotal-issuemaster

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Comment From: pivotal-issuemaster

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Comment From: rstoyanchev

@alan-czajkowski thanks for the pull request but we don't aim to include every possible header name in HttpHeaders.

Comment From: alan-czajkowski

@rstoyanchev this is a pretty important addition to the Content-* list of headers

Comment From: rstoyanchev

I'm not arguing the header is unimportant but rather that we use a different rational by including constants for headers that we actually use in framework code. Create your own constant for other headers you need.